SENSE OF' SIGHT. 645 



In a continuation of the experiments, there was interposed between 

 the spark and the mirror a blackened glass plate, ruled with parallel 

 transparent lines j% of a millimetre in width, and separated from each 

 other by the same distance. The image of this plate, when illuminated 

 by the spark, would appear upon the glass G, so long as the mirror 

 were stationary, as a series of equal alternating black and white lines. 

 With the mirror in motion, if the illumination lasted long enough for 

 the image to be shifted a distance equal to the combined width of a 

 black and white line, these lines would become undistinguishable from 

 each other, as in the case of the revolving disk with black and white 

 sectors. Thus the continuance of the visible lines, under a given rate 

 of motion, proved that the duration of the electric spark was less than 

 a certain calculable period. Their disappearance as distinct objects 

 indicated that the limits of this duration had been reached ; and that 

 it was long enough to allow of the shifting of two adjacent lines. The 

 result showed that the duration of the shortest measurable spark was 

 but little over .00000004 (^ooWo) of a second. 



With a spark of this duration, distinct vision of motionless objects 

 was perfectly possible. The letters on a printed page were plainly to be 

 seen, and even the phenomena of polarization of light distinctly observ- 

 able. It is accordingly sufficient to produce a complete retinal impres- 

 sion. 



These experiments do not indicate the time required for the necessary 

 nervous action in the perception of light. They only show that a lumi- 

 nous impulse having the above duration is sufficient to cause a distinct 

 sensation. But the time which is requisite for the sensation to be per- 

 ceived is very much longer. From the results given in a preceding 

 chapter (p. 431) it appears that the transmission of a luminous impres- 

 sion through the optic nerve, would undoubtedly require at least j^^ 

 of a second, and its perception in the brain considerably more. It fol- 

 lows from this that, at the instant when the image of the electric spark 

 is seen, in the experiment of Prof. Rood, it has, in fact, already disap- 

 peared ; the interval which elapses between its actual occurrence and its 

 perception by the observer being very much greater than the duration 

 of the spark itself. 



The facts detailed above explain the cause of a peculiar optical effect, 

 which has often been observed under the use of the electric spark ; 

 namely, that bodies in rapid motion, if illuminated by an instantaneous 

 discharge, appear to the observer as if at rest. A disk, painted with 

 black and white sectors, if set in revolution under continuous light, 

 appears of a uniform gray; or, if the sectors be painted of the rainbow 

 colors, their tints are mingled and the disk appears white. But if such 

 a disk, revolving in a dark room, be illuminated by the electric spark, 

 it becomes visible for an instant, with its different sectors as distinct 

 from each other as if they were at rest. A jet of water discharged from 

 an orifice at the bottom of a vessel, though transparent in the imme- 

 diate neighborhood of the orifice, is turbid lower down; and by instan- 



